It is known to inject a water based fog into the compressor region of a gas turbine engine to increase the power of the engine. The water is atomised when it is sprayed into the compressor region and forms a fog. The water droplets forming the fog vaporise and extract latent heat of evaporation from the gases in the compressor, thereby cooling these gases. This has a beneficial affect on the power output of the engine. A disadvantage of such a system is that evaporation of the water droplets is not readily achieved and requires onerous nozzle and spray pressure specifications to achieve the required cooling effect.